Stainless steel and method of producing same



Jan. 21, 1958 WAXWE|LER 2,820,708

STAINLESS STEEL AND METHOD OF PRODUCING SAME Filed May 17, 1955 I7 I 1 I I I I (aosi+11cr)xl0 G 7 IIIIIIIIIIIIIIIIIIIIIIII'IIIIIIIII'IIIIIIlIIIIIIIIIIIIIIIIIII IIIIII'IIll SUM OF FERRlTE-FORMING ELEMENTS B R a 9 l0 l2 l3 /4 l5 SUM OF AUSTENITE-FORMING ELEMENTS (2500C+EOMnHISNi+290ONWQ COMPOSITION OF OHROMIUM'NICKEL HEAT HARDENABLE STAINLESS STEEL IN TERMS OF THE SUM OF THE FERRITE-FORMING ELEMENTS (CHROMIUM AND SILICON) VS. THE SUM OF THE AUSTENITE-FORMING ELEMENTS (NICKEL, MANGANESE, CARBON, AND NITROGEN.)

D Inventor James H. Waxweiler By X4 his Attorney STAINLESS STEEL ANDcMETflOD OF PRODUCING SAME James. Hflwaxweiler, Middletown, Ohio, assignor to *Armco Steel Corporation, a corporation of Ohio Application May 17, 1955, Serial No. 508,849 12Claims. .(Cl. 75128) My invention relates generally to the anstenitic stainless steels, and more especially to an ausenitic stainless steel of eriticalicomposition which ishardenable by heat-treatingrmethods, to the hardened'steel and articles fashioned .Of the same, and to the'method of hardening.

One ot-"the objects of my invention is the provision of a stainless steeliwhichis corrosion-resistant, ductile and readily fabricated as by drawing, forming, punching, bending, and the like, and which may be subsequently -hardened.andastrengthened by heat-treating methods to give high-hardness, great strength and substantial tough- -ness,zall".without necessity for special hardening agents such "as. aluminum and copper with their attendant difficulties.

' Another-object is the provision of a simple,- practical and reliablemethod of achievingan austenitic stainless resulting from oxidation during heat-treatment as well as free of distortion in treatment.

A further object is the, provision of various fabricated stainless steel. articles and products, as well as the steel itself, which are hard, tough and strong, with a high ratio of strength 'toweight, both at-vroom temperature and at temperatures approaching 800 or 900 F.

Other objects of. my invention in part will be obvious and inpart pointed out during thecourse of the description which follows.

Accordingly, my. invention residesin the combination of elements, composition of-ingredients, and in thevarious operational steps, and the relation of each of the same to one or more of the others, the scope of the application-of all of which-is set forthin'the claims at the end of thisspecification.

@In-the single figure ofi'thexaccompanying drawing I show the composition limits ofthe steel of my invention and also the limits of the preferred composition.

As conducive to a better understanding of certain features of my-invention it may be noted at this point' that the stainless steels have foundgreat'favor in the arts. Theyare employed in an almost infinite variety of applications where resistance to corrosion, strength and toughness are-required. ySuch steels; generally" contain some to 35% chromium with remainder principally iron. "There, (of course, are present the ingredients carbon, manganese, silicon, phosphorusandsulfur in minor amounts, althoughsubstantial amounts of any one or more ofthese ingredients may be used to achieve known desired vproperties. S0, too, there may, be, includedsmall amounts of nickel, molybdenum, ,tungsten,.titanium-, columbium or "other alloying elements' for special purposes.

Illustrative of-Qthe type of steel noted are the 12% chromium, 17% chromium and :1612% chromium-nickel The quench-hardening operation, however, frequently is found to adversely afiect the ,sfljrface condition. of the steel and oecasicnally'its shape, as well. "Unfortunately, -thesehardening-temperatures are-such-that there-is heavy sealing and metal loss. Moreover, it not infrequently Patented Jan... 21 r1958 "ice ' Z happensjthat .there is a tendency toward some distortion of the. steel as the result of the hightemperatures ofrheattreatment.

Perhaps the most popular type of-stainless-steel is found in the austenitic chromium-nickel ,grades. Such 'a'zsteel in addition to chromium in theamount. of. some 16%t0 30% also contains nickel in large amounts forflexample 5% to 30%. Other ingredients, of course, such as carbon, manganese, silicon, phosphorus .and ,siliconsares-present in minor amounts, or in substantial amounts for-special purposes if desired. And, hereagain, molybdenum, tungsten, 'c'olumbium, titanium and .the like maybe purposely included in. the composition-to achieve particular known benefits.

The austenitic chromium-nickel stainless steels,-.how-

, ever, are not generally consideredto be 'hardenable by heat-treating methods. The hardness had-insheet, strip, various special shapes, rods-and the .like is: achieved by cold-Working methods, that is cold-rolling and cold-drawing, for example.

More recently it has, been'found thatcertain ofqthe chromium-nickelstainlesssteels will respond to a .comparativ ely low, temperature heat-hardening, treatment when there is includedin the. composition of: the steel some strong carbide-formingelement such as columbium or titanium. These ingredients;\however represent special additions which are made-at substantial expense.- Eurther, titanium may in ,somerrinstances detract, from thelclea-nliness of the steel, while columbium is, a.rare andhighly strategic element.

There is a further type, ofchromium-nickel-stainless steel which responds tov hardening treatment alsotgattthe comparatively low temperatures, this byreasonofin- .duced in the production of steels ofrsubstantialtaluminum content. These steels during vteeming-are.inclined Eto scum formationon the molten metal-.stream-which rnakes pouring more difficult.

Accordingly, an object of my inventioniszthe provision of a chromium-nickelstainless steel whichadoessnot .need

" added expensive hardening agents, and whichv steel readily lends itself to fabrication into a host of manufactured articles which are subsequently hardened by heat-treatment at comparatively low temperatures to achieve hardness, strength and great strength to weight ratio. A further object is the provision of a method for consistent- Referring now more particularly to the practice of my invention, I find that by employing particular amounts of the ingedients chromium and nickel with small amounts of carbon, manganese, silicon and nitrogen, and by further providing a highly critical relationship between the in gredients chromium and silicon on the one hand and the ingredients nickel, manganese, carbon and nitrogen on the other, I produce an austenitic chromium-nickel stainless steel which lends itself to hardening by heat-treatment at 'acomparatively low temperature.

I The-steel of my invention is ductile and readily lends itselftoworking and forming by known fabricating methods such as bending, drawing, stretching, forming and the. like. Stainless steel plate, sheet, strip, bars, rods, wire, extrusion shapes and tubes of my stainless steel are worked; formed and otherwise fabricated by known methods into a host of articles of ultimate use. The Working and forming, of course, is performed on the various" products as they come from the steel mill in the annealed condition, annealing being had in the mill "time to achieve the desired hardening effect.

, carbon, manganese and nitrogen. times the percentage of chromium plus 80 times the perat some suitable temperature for example 1700 F. to 2200 F. and preferably 1900 F. to 2000 F. And the articles subsequent to fabrication are subjected to heattreatment at a temperature of some 1200 F. to 1700" F. in order to transform the metal followed by reheating at a temperature of 300 F. to 1150 F. for sufiicient Actually, I find that best results are had with treatment'at a temperature of 700 F. to 900 F., preferably 800 F. to 850 F. The time of treatment ranges from several minutes, say ten minutes, on up to several days time; in

- fact, further appreciable hardening may be observed for times of treatment up to 500hours or more without loss of ductility. In general, I prefer a treatmentof at least hours.

Where desired, I make of' my steel various castings ,to

desired specification; for example, precision castings such as aircraft fittings. And these are hardened by the combination of transformation and heat-hardening treatments noted.

In the steel and method of my invention a ductile metal is realized thereby permitting a highly desirable degree of workability and formability. Moreover, the worked or formed metal is hardened and strengthened by heattreatment at such low temperature that there'is a complete absence of destructive scaling and, of course, treatmenttemperature is so low that distortion is avoided.

More specifically, the steel of my. invention essentiallyconsists of chromium in the amount of 12.0% to 21.0%, nickel in the amount up to 11.5%, carbon upto .20%, manganese up to 5.0%, silicon up to 2.00%,nit1ogen up to .20% and remainder iron. Sulfur and phosphorus, of course, are present in the usual minor amounts,

or larger amounts of these two elementsmay be used where desired. As a further limitation on the composition, however, the several ingredients chromiu'm and silicon bear a particular relation to the ingredients nickel, More' especially 77 centage of silicon representing the sum of the ferriteforming elements bears a very critical relation to the sum of the austenite-forming elements as'defined by 115 times the percent of nickel plus 80 times the percent of manganese plus 2500 times the percent of carbon plus 2900 times the percent of nitrogen. For'best understanding,

the critical range of composition in terms of these values is given in the drawing. The critical range of my steel is defined by the area A, B, C and D. 1

A preferred range of composition, a range in'wh'ich there is had best resultsin terms of a balance'of such 4 factors as resistance to corrosion, ductility to permit ease of forming and strength and hardness achieved. by heattreatment subsequent to forming, essentially consists of 16% to 19% chrominum, 4% to 7% nickel, .05% to .12% carbon, .20% to 4.0% manganese, .20% to 1.0% silicon, .01% to .10% nitrogen, with remainder iron. Here again, of course, it will be understood that the steel may contain the usual impurities such as phosphorus and sulfur in the usual amounts. And as a further. limitation in my preferred composition the relationship between the ingredients chromium and silicon on the one hand and the ingredients nickel, carbon, manganese and nitrogen on the otheris defined by the area of the circle a,'b, c

and d of the drawing. 7

The highly critical character of the composition of the steel of my invention readily is apparent from the physical properties of eight specific compositions of my steel as compared with those of six steels of similar general composition but lacking the critical balance between the chromium and silicon contents on the one hand and the 7 sum of the nickel, carbon, manganese and nitrogen contents on the other. The eight steels of my composition, these in the form of thin strip, are represented by the items 18, 21, 2, l0, 7, 34, 6 and 45 of the following table, the composition of the four steels falling outside of my critical range of composition on the high side being indicated as items 29, 28, 54 and 44, and two compositions falling outside on the low side being indicated as items 20 and 5.

. TABLE I Composition'analysis of fourteen specific compositions of Cr-Ni stainless steel Item N o 0 Mn Si Cr Ni N Remarks 79 .34 18.62 6.13 102 Too Stable. 96 .33 18. 24 6. 14 067 Do.

. 84 67 20. 22 6. 16 011 'Satisfactorv 74 52 18. 45 6. 13 .013 D0.

. 72 56 18. 62 6. 12 010 Too Unstable. 62 50 16. 13 6. 05 030 Do.

All fourteen of the steels of Table I were annealedat 1950 F. followed 'by cooling to room temperature, for example F. Report on the mechanical properties of annealed samplesis given in Table II, column A, below:

TABLE II Mechanical properties of the steels of Table I following i annealing treatment and annealing plus transformation plus hardening heat treatment A p, B

1,950 F. cool to +60 F. A. +1,400 F. min-cool to 30 F.+800 F.-50 hrs-air Item Ult. .2% Yld. Ult. .2% Yld.

Tens Str. Elon. Rock. Tens. Str. Elon. Rock.

1,000 1). s. i. percent ness 1,000 1). s. 1. percent ness p. s. l. p. s. i.

45. 5 66 B89 127. 5 60.0 51. 5 B97 39. 3 43 B87 138. 5 64. 6 36. 5 B99 30. 4 56 B79 120. 5 44. 6 40. 5 B91 23. 0 54 B63 111. 5 50. 1 26. 0 B92 54. 3 47 B94 144. 0 100. 3 21. 0 031 40. 9 18 B90 174. 0 168. 5 9. 5 G39 44. 6 40 B88 190. 2- 177. 8 12. 0 C42 46. 4 23 B93 179. 5 172. 0 9. 5 041 40.0 36 B91 171. 6 155. 4 15. 5 C39 38. 3 31' B89 173. 5 166.5 12.0 040 39. 5 24 B89 .176. 5 159. 2 13.5 C39 24. 6 38 B69 138. 0 96. 2 16. 0 C28 71. 8 10 C35 66.3 10 C24 The composition of the fourteen steels'of'Tables'I'and II expressed in terms of the sum of the ferrite-forming elements (chromium and silicon) versus the sum of the austenite-forming elements (nickel, manganese, carbon a All twelve of the, steels reported, in Table .II, :colun'm Careful comparison, however, of thepropertiesof the and nitrogen) is given in Table'III belowand graphically 5 hardened steels of column B with, the annealedsteelsof presented in the drawing. column A shows that there is ,onlyav slightincreasejn TABLE 1 the yield strength of the steels of items 29, 28,54 and 44. Calculated sum of ferrite-forming elements and'sum of g g s i gz z ffigh Z d 222% a s Z3322? austenite-forming elements of the fourteen specific-comp P g I And 1t will be seen by reference to the figure of the drawposztlons of Table I mg that each of these four steels he outside the.compo- Austenite; sition limits defined by the, area A, B, C and ,D. This 'Femte-Fvrmers Fbrmrs further demonstrates the highly critical composition bal- 1 N. o k mm o (77 ifg l X1 EJ 2501 Rama 8 ance of my steel. 7

' 10- The steels of items 18, 21, 2, 10, 7, 34, 6 and 45,. all falling Within my critical composition range, are seen .to e 31 Toolggfib erespond significantly to the transformation and hardening 12: 2: 7 treatment. In every case the yield strength, isincreased 9.67 14. 68 .Do. 1610 9'88 Satisfactory to at least 90,000 pounds/sq. 1nch. In the case of steels, 14.62 8.92 D6; items 2, 10, 7, 34 and 6, all of WhlCh l1e w1th1n the-pre iii; g 38- ferred range as defined by the circle a, b, c, d, the yield 14: 51 10122 "Do: strengths are much higher. gig J 8% gg- The effect of time and temperature of the hardening 9.56 14; 03 Do. treatment isshown in Table IV below for the steels desiigg 3:33 ggf ignated items 2 and 34 typical analysis.

TABLE IV Mechanical properties of the steel of items 2 and 34 of "Table I when annealed, transformed and hardened, the latter at difierent times and temperatures 0] treatment Ult. Tens. .2%'Yld. E1011. 2" Rock. Heat Treatment Str. 1,000 Str. 1,000 Percent Hardness p. s. i. p. s. i.

Item 2:

1,050 F. +1,400 F. cool to 60 +s00-90 1111.115 156. 2 118. 3 18 032.5 1,950 F. +1,400 F. cool to 60 +80065 hrs. 165. 4 122. 7 17 034. 6

1,950 F. +1,400 F. c661 to 60 +s50-90 mins 157. 0 123.1 19 C83 1,950 F. +1,400 F. 0001 to 60 +90090 mins- 151. 5 114. 2 19 032 1,950 F. +1,400 F. 0001 to 60 +800-10 minsn 175. 2 147. 0 13 C39. 5 1 950 F. +1,400 F. cool to 60 +s0065 his--." 190. 2 177.8 12 042 1,950 F. +1;400 F. 6061 to 60 +90010 ruins.-. 172. 4 152. 3 1a 039 I ng o" F. +1,400 F. 5661 to 60 +1.150-10 mins- 142.1 105. 7 16 C tem 1,950 F. +1,400 F. 0001 to -30 +s00-9 mins 160.8 140.0 10.0 035.5 1,950 F. +1,400 F. cool to -30 +s00-45 mins 163. 2 145. 5 9. 5 036 1,950 F. +1,4o0" F. C001 to -30 +8003 hrs 163.4 149. 7 9. 0 036. 5 -1,950 F. +1,400 F. cool to 30 +s0015 hrs 169.3 158.6 10.0 037.5 1,950 F. +1,40o F. cool to -30 +s005o hrs 173. 5 166. 5 12. 0 039 1,950 F. +1,400 F. cool to -30 +s00120 hrs- 183. 5 175. 8 11.0 040 1,950 F. +1,400 F. cool to -30 +800-240 hrs. 188.0 177.4 11.5 041. 5 1,950 F. +1,400 F. cool m-30 +800-500 hrs 189. 6 178.8 12.0 041. 5

From the results given in Table II, column A, it-will be noted that the steels designated as items 5 and 20 exhibit considerable hardness immediately upon cooling from the annealing temperature. These steels are much too hard in the annealed condition for satisfactory working and forming operations. The Rockwell hardness figures of C24 and and the low elongation values clearly show that the metal had transformed before reaching room temperature and these steels which, as noted in the drawing, are outside the critical range of composition of the steels of my invention are unsatisfactory. Both steels, although falling within the broad range of the chromium and nickel content of the steel of my invention, fall outside the critical range ofcomposition balance as defined by the area A, B, C and D of the drawing.

Inasmuch as the steels of composition, defined by items 5 and 20, are unsuitable because of excessive hardness upon reaching the customer they need be given no further concern. The mechanical properties of the remaining twelve steels, after being subjected to a transformation treatment, notably reheating to 1400 F. for ninety minutes followed by'cooling to --30 F. and then reheating to 800 F. for hours are; shown in Table II, ColumnB.

The samples of the steel of item 2 compositions are seen to develop mechanical properties that are-virtually the same, whether the ageing treatment is performed at 800, 850 or 900 F. Some benefit is seen to be had, however, by prolonged treatment. Thus it will be seen that there is a substantial increase inyield strength, ultimate tensile strength, and in hardness for a treatment at 65 hours as compared to one at minutes. Moreover, samples of steel of item 34 reveal substantial increase in ultimate strength, yield strength and hardness with an increase in the length of time of treatment,- although as a practical matter it appears that substantial maximum properties are achieved in about hours.

A hardening treatment at a temperature of 1150 -"'F. is not seen to develop the full potential of the properties of my steel as indicated by the physical tests on-the final sample of item 2 as given in Table IV above, this temperature of course actually being the top limit ofthe range of my treatment as more particularly described above.

Strength and hardness are had in the steels of my invention eve-n where the hardening treatment is conducted for only a few minutes at hardening temperature. Be st response to short temperature treatments are achiey'ed where the previously transformed steel has been cooled to low temperatures prior to final heat treatment as revealed by comparison of the two samples under item 2 of Table I H3above,'one cooled to 60 F. from 1400 F. flandthe "other to +60 'F., both of which are then finally treated at 800 F; for 65 hours. Whilethe'former sample'isseen'to have a yield strength of, 122,700

latter has developed a strength of 179,000.

Thus, it will be seen that I provide in my invention a chromium-nickel stainless steel of critical composition balance, as well as cast articles of manufacture and converted products such as plate, strip, bars, rods, wire, ex- .trusion shapes and tubes and the like, all of whichare soft, ductile andreadily lend themselves to a variety of working and other fabricating operations, and which subsequently lend. themselves to heat-treatment with comparatively low temperatures to achieve hardness and strength. In the annealedcondition the steel and articles of my invention possessa hardness on the order of RockwellfB 69fto 94, and a yield strength not exceeding about 60,0001), sci. Inthe hardened condition, however, there is h'ad a hardness of at least. Rockwell C28 and a yield strength of atleast 90,000 pas. i. Actually, in the hardened steel and articles of my invention the strength figures approach an ultimate tensile strength of 190,000 p. s. i. and more, and a.2%,,yield strength, of 180,000 p. s. i.

, p..s. i., the

at a temperature of about 1900" to 2000 Rand cooling; reheating ata temperature of 1200 to 1700 F. and coolingto below room temperature; and then hardening "byreheating to a temperature of 300 to 1150"F.

]3. In fthe; productionf pf heat-hardened yaustenitic chromium-nickel stainless steel, the art; which comprises providing a steel essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20%, I manganese "upto 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickeL'manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C, and D of the accompanying diagram; annealing the same and cooling; transforming the same by reheating and cooling; and then hardening by reheating to a temperature of 800 to 850 F. for a timeof a few minutes to several. days.

4. In the production of heat-hardened austenitic chromium-nickel stainless steel, the art which comprises providing a steel essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to 20%, manganese up to 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C and D of the accompanying diagram; annealing the same 1 and cooling; transforming the same by reheating and a cooling; and then hardening by reheating to a temperature of 700 to 900 F. for at least 10 hours.

dition. And the transforming and hardening treatments in accordance with my invention are practiced without fabrication. Moreover, the hardened, finished articles are .free of the directional properties which commonly are encountered in the various austenitic chromium-nickel stainless steels in which hardening is achieved as a result of cold-working operations.

trative embodiments hereinbefore set forth, it will be understood that all matter described herein or shown in the accompanying drawing is to be interpreted. as illustrative and not by way of limitation.

I claim as my invention:

1. In the production of heat-hardened austenitic chromium-nickel stainless steel, the art which comprises providing a steel essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20%, manganese up to, 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickeL'manganeSe, carbon and nitrogen on the other beingsubstantially in accordance with the area A, B, C and D of the accompanying diagram; annealing the same at a temperature of about 17009 to 2200 F. and cooling; transforming the same by reheating at a temperature of 1200 to 1700 F. and cooling; and then hardening by reheating to a temperature of 300 to 1150 F.

2. In the production of heat-hardened austenitic chromium-nickel stainless steel, theart which comprises providing a steel essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20%, manganese up to 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of'said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C

and D of the accompanying diagram; annealing the same danger of excessive scaling of the steel subsequent to 5. In the production of heat-hardened austenitic chromium-nickel stainless steel, the art which comprises providing a steel essentially consisting of chromium 16% to 19%, nickel 4% to 7%, carbon .05% to .12%, manganese 20% to 4.0%, silicon .20% to 1.0%, nitrogen .01% .to .10%, and remainder substantially all iron, with the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the f area A, B, C and Dof the accompanying diagram; annealing'the same and cooling; transforming the same by reheating and cooling; and then hardening the transformed steel by reheating to a temperature of 300 to 1150 F. ,lfor a time of a few minutes to several days.

6. In the production of heat-hardened austenitic I chromium-nickel stainless steel articles of manufacture, the art which comprises fabricating annealed austenitic chromium-nickel stainless steel plate, sheet, strip, bars, rods, wire, extrusion shapes and tubes, essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20%, manganese up to 5.0%, silicon up to 2.00%, nitrogen uptb .20%, and remainder substantially all iron, the relative amounts of said'chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially inv accordance with the area A, B, C and D of the accompanying diagram; transforming the'same subsequent to fabrication by heating to a temperature of 1200? to 1700" F. and cooling; and then hardening by reheating to a temperature of 300 to ;1150 F. j I p 7. In the production of heat-hardened austenitic chromium-nickel stainlesssteel articles of manufacture, the art which comprises fabricating annealed austenitic chromium-nickel stainless steel plate, sheet, strip, bars, rods, wire, extrusionshapes and tubes, essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%,

carbon upto .20%,111anganese up to 5.0%, silicon up to tion by heating and cooling to below room temperature; and then hardening by reheating to a temperature of 700 to 900 F. for at least hours.

8. In the production of heat-hardened austenitic chromium-nickel stainless steel articles of manufacture, the art which comprises fabricating annealed austenitic chromium-nickel stainless steel plate, sheet, strip, bars, rods, wire, extrusion shapes and tubes essentially consisting of chromium 16% to 19%, nickel 4% to 7%, carbon .05% to .12%, manganese, .20% to 4.0%, silicon .20% to 1.0%, nitrogen .01% to .10%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C and D of the accompanying diagram; transforming the same subsequent to fabrication; and then hardening by reheating to a temperature of 300 to 1150 F. for a time of a few minutes to several days.

9. In the production of heat-hardened austenitic chromium-nickel stainless steel cast articles of manufacture, the art which comprises providing a casting essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5 carbon up to .20%, manganese up to 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance With the area A, B, C and D of the accompanying diagram; annealing the same at a temperature of about 1700 to 2200 F. and cooling; transforming the same by reheating at a temperature of 1200 to 1700 F. and cooling; and then hardening by reheating to a temperature of 300 to 1150 F.

10. Heat-hardened austenitic chromium-nickel stainless steel articles of manufacture essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20 manganese up to 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron,

the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C and D of the accompanying diagram; said articles subsequent to fabrication having been transformed by heating at a temperature of 1200 to 1700" F. and cooling and then hardened by heat-treating at a temperature of 300 to 1150 F.

11. Heat-hardened austenitic chromium-nickel stainless steel articles of manufacture essentially consisting of chromium 16% to 19%, nickel 4% to 7%, carbon .05 to .12%, manganese .20% to 4.0%, silicon .20% to 1.0%, nitrogen .01% to .10%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen 011 the other being substantially in accordance with the area A, B, C and D of the accompanying diagram; said articles subsequent to fabrication having been transformed by heating at a temperature of 1200 to 1700 F. and cooling and then hardened by heat-treating at a temperature of 300 to 1150 F.

12. Heat-hardened austenitic chromium-nickel stainless steel cast articles essentially consisting of chromium 12.0% to 21.0%, nickel up to 11.5%, carbon up to .20%, manganese up to 5.0%, silicon up to 2.00%, nitrogen up to .20%, and remainder substantially all iron, the relative amounts of said chromium and silicon on the one hand and nickel, manganese, carbon and nitrogen on the other being substantially in accordance with the area A, B, C and D of the accompanying diagram; said articles having been transformed by heating at a temperature of 1200 to 17 00 F. and cooling and then hardened by heat-treating at a temperature of 300 to 1150 F.

References Cited in the file of this patent Transactions: American Society for Metals, vol. 41, 1949, page 1316.

Transactions: American Society for Steel Treating, vol. 19, November 1931, to June 1932, pages 501 to 507. 

12. HEAT-HARDENED AUSTENITIC CHROMIUM-NICKEL STAINLESS STEEL CAST ARTICLES ESSENTIALLY CONSISTING OF CHROMIUM 12.0% TO 21.0%, NICKEL UP TO 11.5%, CARBON UP TO .20%, MANGANESE UP TO 5.0%, SILICON UP TO 2.00%, NITROGEN UP TO .20%, AND REMAINDER SUBSTANTIALLY ALL IRON, THE RELATIVE AMOUNTS OF SAID CHROMIUM AND SILICON ON THE ONE HAND AND NICKEL, MANGANESE, CARBON AND NITROGEN ON THE OTHER BEING SUBSTANTIALLY IN ACCORDANCE WITH THE AREA A, B, C AND D OF THE ACCOMPANYING DIAGRAM; SAID ARTICLES HAVING BEEN TRANSFORMED BY HEATING AT A TEMPERATURE OF 1200* TO 1700*F. AND COOLING AND THEN HARDENED BY HEAT-TREATING AT A TEMPERATURE OF 300* TO 1150*F. 